Drying kilns are used in the lumber industry for drying cut lumber to a desired moisture content. Generally, these kilns comprise a large drying chamber into which stacked lumber to be dried is placed. The stacked lumber is often supported on rail cars that are movable on a track system in the drying chamber to permit easy and efficient movement of lumber through the kiln.
The drying kiln is generally operated in a batch process, that is, a batch of lumber is moved into the drying chamber, the chamber is sealed and then heated air is blown over and through the stacked lumber in order to remove excess moisture to a pre-determined level. The heated air is mixed with both fresh air from outside the kiln and recirculated air. At the end of the drying period, the heated air is turned off, the drying chamber unsealed and the dried lumber is then removed. The process is then started with another batch of lumber.
There are problems with existing drying kilns dealing chiefly with the circulation of heated air through the drying chamber. There is a tendency for the heated air being directed across the lumber to be unevenly distributed leading to over drying and resultant distortion and even charring of the lumber in some locations and under drying of the lumber in other locations. This problem is also due to inadequate mixing of the heated air with recirculated air which can result in a definite temperature variance across the airstream being blown through the stacked lumber which leads to uneven drying.
Examples of prior art drying kilns for various materials are disclosed in the following patents:
U.S. Pat. No. 1,413,018 to Fujino PA1 U.S. Pat. No. 2,064,965 to Will PA1 U.S. Pat. No. 3,477,139 to Hilderbrand PA1 U.S. Pat. No. 3,646,687 to Berzin PA1 U.S. Pat. No. 3,867,765 to Foster PA1 U.S. Pat. No. 4,014,107 to Bachrich PA1 U.S. Pat. No. 4,098,008 to Schuette PA1 U.S. Pat. No. 4,176,464 to Randolph PA1 U.S. Pat. No. 4,182,048 to Wolfe PA1 U.S. Pat. No. 4,250,629 to Lewis PA1 U.S. Pat. No. 4,485,564 to Iverlund PA1 U.S. Pat. No. 4,862,599 to Brunner and PA1 U.S. Pat. No. 4,955,146 to Bollinger
Some of the foregoing patented designs have been developed specifically to address or avoid the problems of heated air distribution and circulation.
For example, Iverlund discloses a drying kiln that uses microwaves to dry wood instead of heated air and Berzin avoids the problems associated with using heated air by disclosing a wood drying apparatus that subjects wood to ammonia at elevated temperatures and pressures.
Lewis discloses a lumber drying kiln that employs a dehumidifier to treat the heated air being blown over and through the stack of lumber.
Randolph discloses a drying kiln and a method for controlling the operation of the kiln. Randolph relies on a relatively complex arrangement of monitoring the weight of the lumber and the moisture content of the drying air to adjust the drying action so as to control the moisture removal rate.
Foster discloses a drying kiln that employs upper air circulating fans and a central heated air duct.
Bollinger discloses a lumber drying kiln that includes an air treatment and circulating assembly adjacent the top wall that includes a series of reversible fans for distributing air past a series of heating coils.
Schuette discloses a drying kiln that incorporates a fan assembly for directing air to opposite sides of the drying chamber using fans arranged along a longitudinal axis of the chamber. The fans rotate to direct air along a duct and a series of pivoting valves control the direction of air flow.
Wolfe and Bachrich disclose drying kilns that use fans that can be driven in both directions to circulate air to the drying chamber.
Brunner shows a drying kiln that has an arrangement of axial fans mounted for swivelable rotation about a vertical axis.
Hilderbrand discloses a further example of a drying kiln having a swivelable fan to circulate air in opposite directions.
None of the foregoing kiln designs have achieved wide spread acceptance.